1. Field of the Invention
This invention relates to a semiconductor laser device wherein the inrush current is controlled to move the beam position of a laser beam on an emergent face and a driving method for the semiconductor laser device as well as a tracking servo system for an optical disk apparatus wherein a semiconductor laser device is employed as a light source.
2. Description of the Related Art
In order to record information onto an optical disk or optically reproduce the thus recorded information, a laser beam emitted from an optical head must be irradiated at a precise position of a track on the optical disk. To this end, an optical disk apparatus is provided with a tracking servo system.
FIG. 10 schematically shows a tracking servo system of an optical disk apparatus disclosed in Japanese Patent Publication No. Sho 58-55566. A correction method in tracking of a conventional optical disk apparatus will be described subsequently with reference to FIG. 10.
Light emitted from a semiconductor laser 100 is reflected by a half mirror 107 and converged upon an optical disk 104 by a lens 102 fixed on a displaceable cell 103 to form a spot 106. Then, the light reflected from the optical disk 104 is detected by means of a detector 101 to detect a displacement from an object track, and the cell 103 is moved by a motor 109, which is controlled by a feedback circuit (not shown) in response to the thus detected displacement, by way of a bar member 108 interconnecting the motor 109 and the cell 103 to displace the lens 102 in a direction substantially perpendicular to the track to correct the position of the spot 106 on the optical disk 104.
In the tracking servo system, however, since the lens 102 is moved mechanically by the motor 109, the frequency band is limited to several kHz. Consequently, there is a problem in that the speed of rotation of the disk cannot be raised to raise the transfer rate of data.
Meanwhile, another tracking servo system which includes a variable wavelength laser and a diffraction grating is disclosed in Japanese Patent Laid-Open No. Hei 1-224933 or Japanese Patent Laid-Open No. Hei 2-263335.
In the tracking servo system just mentioned, laser light emitted from the variable wavelength laser is collimated once and then diffracted by the diffraction grating so that it is converged on an optical disk. In this instance, since the angle at which laser light is diffracted by the diffraction grating is varied by the wavelength of the laser light, the emergent angle of light from the diffraction grating can be varied by varying the wavelength of the laser to effect scanning of the laser beam on the face of the optical disk.
In the tracking servo systems disclosed in both of Japanese Patent Laid-Open Nos. Hei 1-224933 and 2263335, however, either the refraction index of a waveguide is varied to vary the wavelength or the diffraction grating is mechanically moved to effect scanning of the laser beam. Where the refraction index of the waveguide is varied, since the refraction index in the waveguide is varied readily by an environmental temperature variation or a temperature variation caused by internal heat generation, temperature control must be performed in order to control the wavelength of the variable wavelength laser. This results in the problem of an increase of the size of the tracking servo system. Further, since a diffraction grating is required, there is another problem in that the construction is complicated.
Further, a beam deflection laser wherein the emergent angle of a laser beam is varied is disclosed in Japanese Patent Laid-Open No. Sho 61-236190, No. Sho 61-231786 or No. Hei 3-278490.
The beam deflection laser is based on a twin stripe laser and includes two or more electrodes provided on a semiconductor laser such that the ratio between inrush currents from the electrodes are varied to promote the symmetry of the current density distribution in an active layer to vary the emergent angle of a laser beam. However, even if the emergent angle of the laser beam is varied, the position at which the laser beam is converged on an optical disk by a lens is not moved. Consequently, the beam deflection laser cannot be applied to a tracking system which is required to move a beam spot on an optical disk.